1. Field of the Invention
The present invention relates to a method for growing a nitride single crystal. More particularly, the present invention relates to a method for growing a high-quality nitride single crystal on a silicon substrate, a nitride semiconductor light emitting device using the same and a manufacturing method thereof.
2. Description of the Related Art
A nitride semiconductor light emitting device has been hugely spotlighted in related technological fields as a high-power optical device that ensures full-color emission from light of short wavelengths such as blue or green light. In general, the nitride semiconductor light emitting device is made of a nitride single crystal having a composition expressed by AlxInyGa(1-x-y)N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1.
To manufacture the nitride semiconductor light emitting device essentially requires a technique for growing a high-quality nitride single crystal. However, a substrate material for growing a nitride single crystal thereon, which matches lattice constant and thermal expansion coefficient with the nitride single crystal, has not been commonly available
Chiefly, the nitride single crystal is grown on a hetero-substrate such as a sapphire substrate (α-Al2O3) or a SiC substrate via a Vapor Phase Growth method such as Metal Organic Chemical Vapor Deposition (MOCVD) or Hydride Vapor Phase Epitaxy (HVPE), or a Molecular Beam Epitaxy method (MBE).
However, due to expensiveness and size limited to 2 or 3 inches, disadvantageously, the single crystal sapphire substrate or SiC substrate is inappropriate for mass-production.
Therefore, in the art, a Si substrate which is in common use in the semiconductor industry needs to be adopted. But, owing to differences in lattice constant and thermal expansion coefficient between the Si substrate and a GaN single crystal, a GaN layer suffers too many defects and cracks to be commercialized.
According to a conventional method to overcome this problem, a buffer layer may be formed on the Si substrate, which however is not considered a suitable solution. FIG. 1(a) illustrates a GaN single crystal grown on an AlN buffer layer according to the prior art, and FIG. 1(b) illustrates another GaN single crystal grown on a buffer structure having the AlN buffer layer combined with an AlGaN intermediate layer.
As shown in FIG. 1(a), a GaN single crystal 15 is grown to a thickness of 2 μm on a conventional AlN buffer layer 12 formed on (111) crystal plane of a Si substrate 11. FIG. 2(a) is an optical microscope picture illustrating a surface of the grown GaN single crystal 15 as shown in FIG. 1(a). FIG. 2(a) confirms a number of cracks generated. The cracks occur owing to differences in lattice constant and thermal expansion coefficient which are rarely reduced or narrowed. This disadvantageously degrades capability and lifetime of the device, rendering it almost uncommercializable.
Referring to FIG. 1(b), a semiconductor structure is shown having an AlN buffer layer 23 formed on (111) crystal plane of a Si substrate 21, an AlxGa1-xN intermediate layer 23 grown to a total thickness of 300 nm, with its Al composition ratio (x) varied in a range of about 0.87 to 0.07, and a GaN single crystal 25 is grown on the AlxGa1-xN intermediate layer 23 to a thickness of 2 μm. FIG. 2(b) is an optical microscopic picture illustrating a surface of the GaN single crystal 25 grown in FIG. 1(b). FIG. 2(b) confirms still a great number of, even if reduced from FIG. 2(a), cracks generated. The buffer structure suggested in FIG. 1(b) is not suitable for growing a high-quality single crystal.
Therefore, in the art, there has been a demand for a method for growing a high-quality crack-free nitride crystal layer on a Si substrate and a nitride semiconductor light emitting device using the same.